1. Field of the Invention
This invention relates generally to the field of chemical storage. More specifically, the invention relates to a method and an apparatus for monitoring chemical usage.
2. Background of the Invention
A wide variety of chemicals are used as precursors for chemical vapor deposition in the semiconductor industry. In general, the precursors for the production of films or layers on a wafer are liquids with a relatively low vapor pressure at room temperature (<500 Torr at room temperature). Currently, the precursor materials are stored in steel (stainless, carbon, or other specialty alloy depending upon the precursor characteristics) canisters that are sealed and leak-tight. Canisters may be utilized either close to the chemical vapor deposition chamber, where they are replaced periodically as each canister gets depleted, or in a bulk refill system that provides a source for the automatic refill of canisters at their point of use.
There are at least two reasons why it is important to monitor the chemical level in a storage canister. The first is to ensure that there is adequate precursor chemical in the canister to perform the desired chemical vapor deposition step. The second is to be able to use the maximum amount of the precursor chemical in the process prior to replacing the canister. This second point affects overall system cost effectiveness. Once a canister has been disconnected, any remaining material is discarded. Thus, if a canister is replaced before being maximally depleted, not only are materials wasted, but additional costs are incurred in disposing of the additional waste product.
Various techniques for reducing waste have been attempted, including the use of float switches and level sensors. No method to date has proved entirely satisfactory in actual operations, however.
The fundamental drawback to the technologies that have been currently implemented is that they are not universally applicable to all types of chemicals. For example, float switches may not operate as expected if the specific gravity of the precursor chemical changes. In addition, level sensors typically do not provide a visual indication of the contents of the canister. In order to obtain information, the canister must be connected to the dispensing equipment and the signals from the level sensors must be interpreted and displayed on the dispensing systems computer or a programmable logic controller.
Consequently, there is a need for a method and a chemical storage device that allows real-time monitoring of chemical usage regardless of chemical type or phase.